Switching converters such as DC/DC converters usually provide a regulated output voltage. However, in some applications a regulated output current is required. This is particularly the case when the load to be supplied with electrical energy is current driven. One important type of current-driven loads are light emitting diodes (LEDs) which become increasingly important in the field of illumination devices.
Modern LED-based illumination devices usually include a series circuit of several individual LEDs. Thus, the LEDs “share” a common regulated load current whereas the corresponding voltage drops across the LEDs may vary as a result of temperature variations and aging. Further, the forward voltages of the individual LEDs may significantly differ due to unavoidable tolerances caused by the production process.
For a number of reasons (the most important is efficiency) switching converters providing a regulated output current (load current) are preferred over linear regulators. Load current control, however, requires a load current feedback and thus a load current sense circuit. For this purpose a precise low ohmic sense resistor is usually used. As such a resistor cannot be integrated in an integrated circuit (IC) it has to be provided as an external (i.e., not integrated in an IC) device. Further, a filter circuit may be required to filter the current sense signal (i.e., the voltage drop across the sense resistor) as it is the mean load current which is relevant for the visible brightness of the LEDs. One example for a fully integrated LED driver circuit including control circuitry for operating an appropriate switching converter is the device LM3421 from National Semiconductors (see datasheet LM3421, LM3421Q1, LM3421Q0, LM3423, LM3423Q1, LM3423Q0, “N-Channel Controllers for Constant Current LED Drivers,” National Semiconductor, January 2010).
In view of the existing switching converter control circuits that provide a regulated output current there remains a need for improvement, particularly for integrated control circuits that require fewer external components which cannot be readily integrated in one or more semiconductor chips provided in a one single chip package.